It is known that to prepare express coffee it is necessary to pass hot water at a temperature above 80° C. through ground coffee, at a pressure above atmospheric pressure, preferably comprised between 7 and 9 atmospheres. Therefore, all the soluble components that provide the taste and flavor are extracted from the coffee.
The most characteristic component that differentiates the machines of express coffee from other machines for preparing coffee is the pressure pump, which permits to eject the water at the previously cited pressure, through the infusion chamber that comprises the dose of ground coffee.
Machines that present the particularity that the water pressure to obtain the express coffee is provided by alternative systems to the pressure pump have been developed for obtaining autonomous machines, in which the coffee can be prepared without consumption of electric energy.
International patent WO/2007/088309 discloses one of said machines, in which compressed gas is used instead of a pressure pump. Said compressed gas is injected on the hot water to force the passage of a preset quantity of said water through the infusion chamber. In this machine, that is marketed under the name of “Handpresso”, the gas chamber is pressurized until it acquires the desired pressure by manual pumping of air contained in a cylinder. Then, the compressed gas is released to the receptacle that contains the water dose, to force the water passage through the infusion chamber.
In the machine disclosed in “Handpresso” patent, the pressure that the gas exerts on the water decreases during the distribution, as the water leaves space to the gas. Therefore, the pressure exerted by the gas on the water is not constant throughout the coffee distribution, resulting in a sub-extracted express or low density coffee, therefore of lower quality.
European patent EP0231156 discloses another machine for preparing coffee in which the pressure necessary to feed the water is obtained, in this case, from a plunger that accumulates elastic potential energy from the compression force of a spring placed at one end of the cylinder chamber defining said plunger.
The spring compression is carried out manually pumping water inside the chamber of the cylinder itself, with the objective of retracting the plunger to the active position. The reaction force of the spring on the plunger makes possible to keep a pressure on the water when the plunger moves inside the water chamber of the cylinder, to the rest position.
However, the configuration of the machine disclosed by said European patent has a lot of drawbacks.
One of said drawbacks is the fact that the water is fed through a tube network that connects the water chamber of the cylinder with the infusion chamber. In practice, the long path that the water must cover to reach the infusion chamber prevents the provision of a more or less constant pressure by said spring, which should have physical features to provide said pressure that would prevent it to be housed inside a device with reduced dimensions. Therefore, even in the case that it works, it would produce an express coffee of lower quality.
Another drawback is the fact that the cylinder-plunger assembly that feeds pressurized water is placed inside a deposit that stores the same water than afterwards is fed to the infusion chamber. This is not suitable from a sanitary point of view, and it affects negatively the maintenance of the coffee maker.
On the other hand, as a great number of elements are in contact with the water, even if boiling water is provided to said deposit, said configuration would prevent a suitable temperature of the water to be kept to obtain a coffee infusion in express quality.